1. Field of the Invention
The present invention relates generally to mobile telephone systems, and more particularly to a gain control method and system for mobile telephone systems. The present invention is most applicable to gateways using code division multiple access (CDMA) modulation techniques where power conservation is critical.
2. Description of Background Art
Mobile telephone systems allow customers to place telephone calls from wireless devices referred to as mobile telephones or subscriber units. The mobile telephone transmits the signal to a gateway. The gateway is interconnected to a mobile telephone switch. The mobile telephone switch interconnects the gateways to each other and to public switched telephone networks (PSTNs).
One method that is used for mobile telephone transmission to a gateway is via a ground-based antenna that operates in UHF band. This is the same band used for broadcast television transmission. Use of this method limits the subscriber to communication within a cell which is the serving area to which the antenna can transmit using UHF band. Subscribers can move from cell to cell because hand-offs are possible from one cell to another. However, if no ground-based antenna is within a distance that can be reached using UHF band, such as in a rural area, a subscriber cannot use the mobile telephone.
Developments in mobile telephone system technology have led to mobile telephone systems that can transmit using a low earth orbit (LEO) satellite system, such as the Globalstar LEO satellite system. The mobile telephone systems that use LEO satellite systems can transmit to rural areas because the subscriber does not need to be within close range of a ground-based antenna. As a result, mobile telephone systems using LEO satellite systems are not limited to major cities as are mobile telephone systems that use antennas operating in the UHF band.
The transponder is the component in a satellite that receives and transmits signals from subscribers using mobile telephones. A satellite transponder must be able to carry a large number of subscribers simultaneously in order to be cost effective. Various satellite access schemes such as time division multiplex access (TDMA) and code division multiplex access (CDMA) allow simultaneous access to transponders by a large number of subscribers.
Digital CDMA is preferable to other satellite access schemes as more customers can be carried at a lower cost and higher quality. Low powered signals allow transmission of CDMA signals via small, inexpensive antennas requiring less expensive earth station and network equipment than other satellite access systems. However, because signal power is low, the power must be used efficiently. CDMA systems have low noise and interference because the gateways transmit using low powered signals.
In a CDMA system each customer is carried on an individual channel. CDMA systems modulate and interleave the individual channels so that a large number of channels are spread throughout the same waveform. As a result, multiple customers or users simultaneously share the same subbeam which is referred to interchangeably herein as a narrowband channel or a carrier. A subbeam or narrowband channel is typically approximately 1.23 MHZ in bandwidth.
Because multiple customers or users share the same subbeam, if one customer""s or user""s signal is transmitted at a higher power than the signals of the other customers or users on the channel, interference may occur which may result in unacceptable performance unless the number of users on the subbeam is reduced. In addition, lower power transmission helps overcome fading because signals can be spread through more of the subbeam and more capacity is available in the subbeam for diverse paths. Also, lower power transmission conserves power at the gateway. However, if the power of a customer""s signal becomes too low, the quality of service for that customer becomes unacceptable.
For transmission via satellite, individual subbeams are modulated together to create one wideband channel. A wideband channel comprises 104 subbeams and has a bandwidth of 160 MHZ. However, the number of subbeams that may be carried by a wideband channel""s ability is dependent on the power of each subbeam. The power available to transmit user traffic is the power that the satellite is capable of transmitting less the overhead power required for satellite operation. The number of users that may be transmitted essentially equals the power available to transmit user traffic divided by the power required for each individual user. Thus, the number of users that may be provided service is increased by maintaining overhead power levels and each individual user""s channel at the minimum levels needed for optimum performance. This can be accomplished by limiting the power of each subbeam to the power necessary for high quality transmission. Control of the power of the subbeams and the wideband channel is needed to limit the power of each subbeam and wideband channel to the power needed for high quality transmission and to ensure efficient use of power which allows the maximum number of subbeams, and individual channels, to be carried on a wideband channel.
In addition, an accurate accounting of the power being consumed on the satellite is needed to manage the health of the satellite and reduce costs of satellite transmission. If power demands on a satellite exceed design expectations, satellite batteries will be relied on as energy sources more often than planned during satellite design. Additional demands on satellite batteries will require that the batteries be replaced more often at a cost to the satellite service provider. Satellite service providers often charge for the satellite power consumed. As a result, satellite service consumers need to track accurate measurements of their power demands on the satellite to maintain the power demands to the minimum level needed to provide quality service in order to reduce costs.
The present invention is a novel and improved method for the control of the gain of individual narrowband channels using a wideband power measurement. The system of the present invention is a transmit power tracking loop which controls the power by adjusting the gain applied to the transmitted signal.
Gain control by the transmit power tracking loop provides modifications in the gain applied by a variable gain amplifier based on feedback of power measurements before and after the signal is amplified by the variable gain amplifier. The power measurements include estimations of the power of each individual subbeam prior to amplification and an estimation of the power of the wideband channel after amplification. The power estimates of each individual subbeam are summed for the comparison with the power of the wideband channel.
Open loop gain control is also provided by controlling gain applied by the variable gain amplifier using open loop commands. Open loop commands are generated by an algorithm that calculates the proper gain that should be applied by the variable gain amplifier at regular intervals, such as one second. The algorithm for generating open loop commands calculates the gain that should be applied using the elevation of the antenna, the gain of the dish of the antenna, and various constants. Open loop commands are implemented by adjustments in gain applied by the variable gain amplifier and adjustments of gain applied by each subbeam""s individual modulator.
In order to maintain an accurate accounting of the power used by the satellite, power levels at the gateway are stored and used to provide estimations of the power used by the satellite. The power estimates of each individual subbeam and of the wideband channel are sent to a ground operations control center to be stored for use by other processes to provide analysis, such as of the power consumed by the satellite. The gateway power measurements enable the satellite service consumer to obtain an accurate accounting of the power being consumed on the satellite. An accurate accounting of the power being consumed on the satellite enables the satellite service consumer to maintain the power demands at the minimum level needed to provide quality service and reduce costs. In addition, satellite service providers may use this information to manage the health of the satellite and reduce costs of satellite transmission.
Gain control balances the need to transmit at a low power to utilize capacity for as many customers as possible, avoid overdriving the satellite, and avoid violating flux density limits while maintaining sufficient power for each subbeam to provide high quality service to the users carried on the subbeam. Gain control also allows for adjustment to conditions at the gateway.